In industrial-scale chemical operations, components are used in the feedstream which typically contain low levels of impurities even after being subjected to a purification process. The low levels of impurities still remaining in the feedstream can act as catalyst poisons in the reaction process, adversely affecting the performance of the catalyst. Of particular concern are trace sulfur, halogen, phosphorous, arsenic and selenium impurities that may be present in the feedstream. Metal or noble metal catalysts are generally susceptible to catalyst poisoning by these elements, for example many metals are known to form sulfides even if sulfur is present in the feedstream in quantities below the parts per million level. Processes which use a metal or noble metal catalyst include, but are not limited to, ammoxidation reactions, dehydrogenation reactions, catalytic reforming reactions, and oxidation reactions, in particular partial oxidation of an olefin to form an olefin oxide such as ethylene oxide. These reactions are typically highly exothermic and generally performed in a vertical shell-and-tube heat exchanger comprising a multitude of reaction tubes, each containing a packed bed of solid particulate catalyst and surrounded by a heat exchange fluid. In the production of olefin oxides, such as ethylene oxide, silver-based catalysts are used to convert ethylene and oxygen into ethylene oxide. These silver-based catalysts are especially susceptible to catalyst poisoning even at impurity amounts on the order of parts per billion levels. The catalyst poisoning impacts the catalyst performance, in particular the selectivity or activity, and shortens the length of time the catalyst can remain in the reactor before having to exchange the poisoned catalyst with fresh catalyst.
Typical sulfur-containing impurities present in the feedstream can include, but are not limited to, dihydrogen sulfide, carbonyl sulfide, mercaptans, and organic sulfides. Typical halogen-containing impurities present in the feedstream can include, but are not limited to, freons or halohydrocarbons. In an epoxidation process, additional impurities may include, phosphorous, arsenic, selenium, acetylene, carbon monoxide. The sulfur, phosphorous, arsenic, and selenium impurities present in the feedstream may originate from a hydrocarbon such as an olefin or a saturated hydrocarbon such as methane or ethane. The halogen impurities present in the feedstream may originate from an oxygen source such as air or high purity oxygen.
Over the years, much effort has been devoted to improving the olefin epoxidation process. Solutions have been found in various improved reactor designs.
For example, U.S. Pat. No. 6,939,979 describes the use of an alkali metal treated inert as a diluent for the catalyst positioned in an upper section of the reactor tubes. Treating the inert with an alkali metal reduces the degradation of ethylene oxide by the inert thereby improving the selectivity to ethylene oxide. However, placing an inert material upstream from the catalyst does not significantly reduce the amount of impurities present in the feed which can poison the catalyst.
Thus, not withstanding the improvements already achieved, there exists a desire for a reactor system and reaction process that further improves the performance of the catalyst, in particular the duration of time the catalyst remains in the reactor before exchanging with a fresh catalyst.